1. Field of the Invention
The present invention relates generally to coaxial terminations used to terminate ports that are adapted to receive coaxial cable connectors, and more particularly, to an improved coaxial termination that offers protection against high-voltage surges.
2. Description of the Related Art
RF coaxial cable systems are well known to those in the cable television industry for distributing radio frequency signals to subscribers of cable television service, and more recently, voice and data telecommunications services. The coaxial cables used to route such signals include a center conductor for transmitting a radio frequency signal, and a surrounding, grounded outer conductive braid or sheath. Typically, the coaxial cable includes a dielectric material surrounding the center conductor and spacing it from the grounded outer sheath. The diameter of the center conductor, and the diameter of the outer conductor, and type of dielectric are selected to produce a characteristic impedance, such as 75 ohms, in the coaxial line. This same coaxial cable is sometimes used to provide AC power (typically 60-90 Vrms) to the equipment boxes that require external power to function. Approximately 80% of the cable in a system will carry this AC power.
Within such coaxial cable systems, such coaxial lines are typically coupled at their ends to equipment boxes, such as signal splitters, amplifiers, etc. These equipment boxes often have several internally-threaded coaxial ports adapted to receive end connectors of coaxial cables. If one or more of such coaxial ports is to be left “open”, i.e., a coaxial cable is not going to be secured to such port, then it is necessary to “terminate” such port with a coaxial termination that matches the characteristic impedance of the coaxial line (e.g., a 75 ohm termination). If such a coaxial termination is omitted, then undesired reflected signals interfere with the proper transmission of the desired radio frequency signal.
Coaxial terminations of the type described above are known and available. Typically, such known coaxial termination devices include a metallic outer body which, at a first end thereof, is provided with external threads for mating with the internal threads of a coaxial port on the equipment, box. A center conductor passes through a dielectric secured within the metallic outer body from the first end of the coaxial termination device to an opposing second end thereof. At the second end of the coaxial termination device, a resistor corresponding to the characteristic impedance of the coaxial line is secured, and is coupled between the center conductor and the grounded metallic outer body. If the coaxial line carries AC or DC power, then a low frequency blocking capacitor is typically used to couple the aforementioned resistor to ground. The resistor and capacitor of such known coaxial termination devices are often located outside the controlled characteristic impedance environment, creating an impedance mismatch that reflects some of the forward-transmitted signal back toward its source. These reflections can result in loss of power transfer and interference with, or corruption of, the signal. Accordingly, some signal degradation results from the use of such coaxial termination devices. The degree of such signal degradation at a given frequency, resulting from such impedance mismatch, is sometimes expressed as the RF return loss performance of the coaxial system.
Moreover, when deployed in the field, as in cable TV systems, for example, these known coaxial termination devices can be subjected to power surges caused by lightening strikes and other events. These power surges can damage or destroy the resistive and/or capacitive elements in such a termination, rendering it non-functional. A commonly used surge test, ANSI C62.41 Category B3, specifies that a 6000 Volt open circuit/3000 Amp short circuit surge pulse be injected into the coaxial termination device. At least some of the known coaxial termination devices have difficulty complying with such surge test. Indeed, efforts to make the resistive and capacitive components larger, in order to withstand such power surges, can have the negative impacts of increased costs and/or creating a larger impedance mismatch, and hence, causing poorer levels of RF Return Loss performance. One approach to designing a termination that can withstand the previously mentioned 6,000 Volt surges would be to use a 6,000 Volt capacitor and a high power resistor. Unfortunately, such components are relatively expensive and have a much larger physical size, which tends to increase the size and cost of the housing necessary to contain such components, thereby resulting in a much bulkier and more costly design.
Accordingly, it is an object of the present invention to provide a coaxial termination device capable of maintaining high levels of RF Return Loss performance.
It is a further object of the present invention to provide such a coaxial termination device capable of withstanding power surges without damage to the resistive and/or capacitive elements thereof.
A further object of the present invention is to provide such a coaxial termination device that can simultaneously withstand such power surges without damage, while still maintaining high levels of RF Return Loss performance.
A still further object of the present invention is to provide such a termination device that is relatively compact and inexpensive to manufacture.
Another object of the present invention is to provide such a coaxial termination device that reduces reflection by disposing the resistive component thereof in a controlled characteristic impedance environment.
Still another object of the present invention is to minimize the length of the path between the resistive component of the coaxial termination device and ground (i.e., through the capacitive component) to further minimize inductance and signal reflection.
Yet another object of the present invention is to provide such a coaxial termination device which allows the resistive and capacitive components thereof to be relatively small in size to maintain high levels of RF Return Loss performance while still being able to withstand power surges without damage.
These and other objects of the present invention will become more apparent to those skilled in the art as the description of the present invention proceeds.